1. Field of the Invention
The present invention relates to a transfer device for a plate-like transfer target, e.g., a semiconductor wafer, which is used in, e.g., a semiconductor manufacturing process.
2. Description of the Related Art
In a semiconductor manufacturing system, wafers are transferred among process stations and stored while they are housed in a container (usually called a "carrier" or "cassette") made of, e.g., Teflon, and having upper and lower open ends and many slots. At the process stations, a container (to be referred to as a "carrier" hereinafter), in which, e.g., 25 wafers are housed parallel to each other with predetermined gaps while they are supported in slots, is loaded on an unloading/loading boat. Then, at this position or after the carrier is moved to another place, the wafers in the carrier are sequentially transferred by a wafer transfer device to a predetermined place, e.g., a wafer boat (in annealing) or a load-lock chamber (in vacuum processing).
To transfer the wafers in this manner, while the carrier is set upright, i.e., while the wafers are arranged horizontally, the fork of the transfer device is inserted in each slot through an opening in the front wall of the carrier, and each wafer is taken up by the fork. In this case, the wafers must be accurately housed in the carrier. However, when the carrier is placed on a carrier stage, the wafers sometimes partly project from the opening of the carrier due to the impact of placing the carrier on the carrier stage. For example, if a wafer is tilted in the slot and is thus held unstably, this wafer particularly tends to project. If the carrier is left on the carrier stage for a long period of time, some wafers sometimes project from the carrier due to the vibration caused by an air-blowing fan or the like.
When a wafer projects from the carrier, since the fork of the wafer transfer device takes up a portion of the wafer dislocated from a predetermined position, the wafer sometimes drops from the fork at this time. A dropped wafer cannot be used as a product. If a wafer is damaged, fragments are scattered and cumbersome cleaning becomes necessary. In addition, the manufacture line is stopped, and the entire throughput decreases.
Even if a wafer does not drop from the fork, when, e.g., wafers are to be transferred to groove portions between the pillars of the wafer boat of an annealing device, as the holding region of each groove portion is narrow, a wafer sometimes drops during transfer from the fork to the wafer boat. Accordingly, in order to transfer wafers with a high reliability, projection of a wafer in the carrier must be detected, and upon detecting projection of a wafer, the transfer device must be stopped immediately, and this fact must be informed to the operator.
Furthermore, when wafers are to be taken out from the carrier, in addition to projection of a wafer, the presence/absence of a wafer at each stage (each slot) in the carrier must be detected. More specifically, a predetermined number of wafers may not always be conveyed to a process station while they are housed in a carrier with a predetermined order. Sometimes an accident may happen at a process station of a previous stage, or a testing wafer may be pulled out from the carrier in order to perform inspection of the process of a previous stage. If there is a stage in the carrier in which a wafer is absent in this manner, the system controller must obtain information as to stages in which wafers are present and those in which wafers are absent, and must inform this information to the wafer transfer device.
As a technique for detecting the housing state of wafers in the carrier as described above, a technique as disclosed in, e.g., Jpn. Pat. Appln. KOKAI Publication No. 4-75362 is known. According to this technique, light-emitting and light-receiving elements are arranged above and below a carrier so as to form an optical path at a position opposing the opening in the front wall of the carrier placed on a carrier stage. When a wafer blocks the optical path, it is determined that a wafer projects. At the same time, light-emitting and light-receiving elements are arranged in front of and behind the carrier. When the optical path is blocked upon vertically moving the carrier stage, it is determined that a wafer is present at a stage that blocks the optical path.
In the above detection unit, in order to detect projection of a wafer, the light-emitting and light-receiving elements are arranged to vertically sandwich the carrier and, in order to detect the presence/absence of a wafer, the light-emitting and light-receiving elements are arranged to horizontally sandwich the carrier. Therefore, a space necessary for arranging the carrier becomes undesirably large.
Furthermore, since the thickness of a wafer as a detection target is as small as about 0.7 mm, the heights of the optical axes of the light-emitting and light-receiving elements must be adjusted to coincide within this small allowable range, and it is difficult to adjust these heights.